The present invention relates to a fault detector for an anti-parallel thyristor.
Specifically, the present invention relates to a fault detector for detecting a fault of a thyristor level connected to a static var compensator (SVC). A thyristor valve used in the SVC may be connected in anti-parallel. Additionally, the SVC may be applied to a high voltage direct current (HVDC) system.
The HVDC is a method of power transmission. The HVDC system converts high voltage AC power generated by a power generator into high-efficiency high voltage DC power by using a power converter and transmits the DC power. Thereafter, the DC power is converted into the AC power in a desired area by using a power converter. As compared with high voltage alternating current (HVAC), the HVDC system may be advantageous to long-distance power transmission due to low power loss.
A valve type of the power converter used in the HVDC system may be classified into a current source scheme based on a thyristor and a voltage source scheme based on an insulated gate bipolar mode transistor (IGBT). The current source scheme started to be employed in an HVDC system in the early 1980s and is widely used for commercial purposes till now. On the other hand, the voltage source scheme started to be commercially employed in an HVDC system in the early 2000s and the capacity thereof tends to increase. However, the capacity of the voltage source scheme is small as compared with the current source scheme, and thus, the voltage source scheme is mostly used to connect a large-scale offshore wind farm to an AC power network.
Among a plurality of components included in the HVDC system, the most important component is a power converter which converts DC into AC and converts AC into DC. Since the power converter has a very high operating voltage, a plurality of thyristors are connected in series to constitute one valve. Before the power converter is installed, it is necessary to check the operations of these valves with a voltage and power to be applied during operations. However, the checking of the operations with the voltage and power to be applied during operations consumes considerable power and may cause safety problems.
Meanwhile, the SVC implements a reactive power control function of a synchronous condenser, which is a rotor, in a static type by using a thyristor valve. The SVC is a type of a flexible AC transmission system (FACTS) and may be an apparatus which enables voltage adjustment of a transmission system, transient stability improvement, and the like.
The SVC may be an apparatus which is connected in parallel to a power system and constantly maintains a voltage or performs a desired control operation through absorption or supply of reactive power.
The SVC may be implemented by combining a thyristor controlled reactor (TCR) which controls a phase of a reactor by using a thyristor, a thyristor switched capacitor (TSC) which switches a capacitor, and a fixed capacitor bank, according to intended use.
Thyristor valves used in the TCR and the TSC may be connected in anti-parallel.
The thyristors may become defective due to unintended overvoltage, impulse overcurrent, or the like. Generally, a resistance of a defective thyristor may be 0 Ω, and a defective thyristor may be easily detected through a multimeter.
However, when one of anti-parallel thyristors is referred to as a first thyristor, the other is referred to as a second thyristor, and internal resistances of the first and second thyristors are respectively R1 and R2, a combined resistance (R) is R1*R2/(R1+R2). Thus, if one of R1 and R2 is 0 Ω, the combined resistance (R) is 0 Ω. From this, the defective thyristor can be determined from the first and second thyristors.
That is, due to the structure of the anti-parallel thyristors, in order to determine the fault of the defective thyristor, there may be an inconvenience of having to separate the anti-parallel thyristors and determine the respective thyristors by using a multimeter.